The present invention relates to a photocurable mixture which comprises a diazonium salt polycondensation product or an organic azido compound as a photosensitive compound, and a polymeric binder, and which is especially suitable for the production of printing plates and photoresists.
DE 20 24 244 (=U.S. Pat. No. 3,867,147) discloses photosensitive recording materials comprising cocondensation products of diazonium salts capable of condensation and other, non-photosensitive compounds, capable of condensation, if appropriate combined with polymeric binders, for producing printing plates. These photosensitive materials are distinguished by their high photosensitivity and long print runs. The number of prints obtained is particularly high if water-insoluble binders, for example, polyvinyl formal, are used, but such layers can only be properly processed by means of developers containing a considerable amount of volatile organic solvents. For ecological reasons it is desirable to develop printing plates of this type with purely aqueous solutions. In the most favorable cases, the mentioned printing plates can be developed with relatively aggressive acidic or alkaline purely aqueous solutions by suspending the non-image areas of the layer in the developer, in the form of flakes or relatively small particles. There is, however, the danger of uncontrolled redeposition of flaky matter in non-image areas of the plates whereby the plates are rendered unusable. DE 31 30 987 describes similar mixtures which contain styrene/maleic acid anhydride copolymers as the binders and which can be developed with purely aqueous alkaline solutions. The printing plates obtained using these mixtures, however, yield print runs which are inferior to those of printing plates containing binders which are insoluble in aqueous-alkaline solutions.
DE 30 36 077 (=U.S. Pat. No. 4,387,151) discloses corresponding mixtures which contain binders comprising polymers with alkenylsulfonylurethane side groups. Layers prepared from these mixtures can also be developed by means of purely aqueous solutions. However, when the mixtures are processed to form lithographic printing plates, they yield light-cured coatings whose ink acceptance is unsatisfactory.
DE 24 29 251 discloses a mixture comprising a photosensitive diazo compound and a binder, the binder being obtained by reacting a polymer containing hydroxyl groups or carboxyl groups with a crosslinking agent containing isocyanate groups, epoxide groups or acid anhydride groups. By the crosslinking reaction the solubility of the polymer is reduced. Where these mixtures give materials of high print runs, development cannot be performed without an addition of organic solvents. Products crosslinked to a lesser extent often contain unreacted crosslinking groups and hence have a limited shelf life.
EP 152,819 discloses photosensitive mixtures comprised of diazonium salt polycondensation products and binders containing carboxyl groups, which are obtained by reacting hydroxyl group-containing polymers with acid anhydrides. Plates produced using these mixtures can be developed with aqueous-alkaline solutions, and the mixtures are suited for the production of lithographic printing forms giving high print runs. It is, however, desirable to achieve even better print runs. For this purpose, the publication proposes the addition of minor amounts of other polymers, for example, polyurethanes, in order to increase the abrasion resistance. However, this combination results in a poorer copying performance, such as undesired dot gain. Furthermore, the developing behavior and shelf life of the plates are impaired. Like other mixtures prepared with carboxyl-containing binders, these mixtures also suffer from the draw-back that upon being processed with developers based on tap water there is the risk of the formation of precipitates in the form of sparingly soluble calcium salts, which are deposited developing machines and cause disturbances.
Mixtures of the above generic type, where polyurethanes are employed as polymeric binders, are known from U.S. Pat. No. 3,660,097 and DE 27 39 774. It is a disadvantage of these mixtures that the binders are sparingly soluble in the solvents conventionally used for coatings and that the solutions have to be filtered several times to remove insoluble residues. The developability of these mixtures in aqueous-alkaline media is very limited and the print runs are inadequate for high-performance machines.
A similar mixture is known from EP 30,001. The binder contained therein is a branched polyurethane. Using this mixture, relatively high print runs can be achieved, but there are still a number of drawbacks. For example, acidic aqueous developer solutions with an addition of organic solvents are preferably employed for achieving a scum-free, rapid development of the photocured layer. When used for automatic processing, these developers may cause corrosion problems at the light metal parts of the processing equipment.
In DE 37 32 089 graft polymers are described, which comprise a polyurethane as the graft backbone and grafted-on vinyl ester units, which are at least partially saponified to give vinyl alcohol units. The polymers are suitable as binders for pigments; for preparing printing inks, thermoplastic adhesives and solvent-containing adhesives; as constituents of varnishes or coatings for fibers, films and metals, and for thermoplastic shaped articles.
JP 246,047/87 describes photopolymerizable mixtures wherein graft polymers of polyurethanes and polyvinyl alcohol with mercapto groups are contained as binders.
Graft polymers obtained from the graft polymers mentioned above by acetalizing with aldehydes, are described in German Patent Application P 38 35 840.9.
German Patent Application P 38 24 146.3 discloses photocurable elastomeric mixtures which contain a compound polymerizable by a free-radical process, a photoinitiator and, as the binder, a graft polymer according to DE 37 32 089, which is soluble or dispersible in an aqueous solution.